It is known for a long time to prepare an aromatic hydroxycarboxylic acid by Kolbe-Schmitt reaction. Although German Patent No. 663,774 discloses an example of a process for preparing 2-hydroxynaphthalene-3,6-dicarboxylic acid from 2-hydroxynaphthalene-6-carboxylic acid, this process is not a method wherein dicarboxylic acid can be prepared directly from 2-hydroxynaphthalene.
Although Japanese Patent Publication (KOKAI) No. 197244/1982 discloses a process for preparing 2-hydroxynaphthalene-3-carboxylic acid (preferred reaction temperature is described as 240-280.degree. C.), it does not refer to the preparation of 2-hydroxynaphthalene-3,6-dicarboxylic acid.
Japanese Patent Publication (KOKAI) No. 340581/1994 discloses a process for preparing monohydroxynaphthalene carboxylic acid (2-hydroxynaphthalene-3-carboxylic acid and 2-hydroxynaphthalene-6-carboxylic acid) from alkali metal naphtholate by Kolbe-Schmitt reaction. Small amounts of 2-hydroxynaphthalene-3,6-dicarboxylic acid are produced as a by-product in this process wherein a reaction pressure is low (about 10 bar).
A process for preparing 2-hydroxynaphthalene-6-carboxylic acid is described in Japanese Patent Publication (KOKAI) No. 146843/1988. Although characteristic of this process is that the reaction is carried out under pressurized carbon dioxide while the formed water is successively removed from the reaction system, the pressure of the reaction system is 20 kg/cm.sup.2 (G) at the most, and 2-hydroxynaphthalene-3,6-dicarboxylic acid is not produced.